1. Field of the Invention
The invention relates to an elongated radioactive element to be attached to an end of an elongated wire-shaped element.
2. Description of the Prior Art
One application of such a radioactive element is its use in the event that local irradiation is desired, particularly, if the place of irradiation is not directly or readily accessible, for instance in the event of a tumor present in a body. In such cases, hollow needles for instance in the case of a mammary tumor, or hollow catheters for instance in the case of a uterine tumor are used within which the radioactive element can be pushed into the desired position for radiation treatment. In order to protect the persons performing the treatment from ionizing radiation, the radioactive element is contained in a shielding housing which is connected by a tube to a needle or catheter. The radioactive element is attached to an end of a wire-shaped element so that the radioactive element can be brought, by remote control, out of the shielding housing through the tube into the desired position for irradiation by the displacement of the wire-shaped element by a drive unit.
Another use also known as intravascular brachytherapy of such a radioactive element is use in the treatment against restenosis by neo-intima proliferation of a blood vessel after a recanalization treatment. Such a treatment of blood vessels around the heart is known, inter alia, as percutaneous transluminal coronary angioplasty (PTCA; in the case of blood vessels around the heart), and atherectomy. Upon such a treatment, a substantially occluded blood vessel (for instance as a result of the deposition of so-called plaque in the lumen of the blood vessel) is stretched by means of an expandable element such as a fluid-inflatable balloon (angioplasty balloon), fastened to an elongated element such as a catheter, in order to permit the blood to flow again substantially unimpeded through the stretched blood vessel. In certain cases, a so called "stent" (metal wire stretcher) is inserted in order, among other things, to prevent "elastic recoil".
In a large number of cases, a new recanalization treatment is found necessary after a relatively short period of time since a constriction again forms in the blood vessel or has already formed. The constriction may be a result of tissue (known as neo-intima hyperplasia or neo-intima proliferation) developing at the stretched place, probably because the wall of the blood vessel was damaged by the stretching. There are strong indications that this formation of tissue can be avoided or at least reduced to a great extent if, during or shortly after the recanalization treatment, the blood-vessel tissue concerned is irradiated with ionizing radiation, in particular .beta. and/or .gamma. radiation, so that a subsequent recanalization treatment is no longer necessary, or in any event a much longer period of time elapses before such a treatment is necessary.
In medical applications of radioactive sources for providing the ionizing radiation mentioned above, radioactive materials are used with half-lives suitable for medical applications. Depending on the application, this half-life is at least a few days. Moreover, for use in intravascular brachytherapy the radiation used should have an average energy E.sub.mean of at least 0.6 MeV. E.sub.mean is also referred to in the following as .beta.-energy. To this day, choosing a suitable radioactive material meant browsing through voluminous books with extensive tables indicating for each element of the periodical system, whether it is radioactive or not, and if so, in what ways the radioactive element may decay. Thus all radioactive elements whose decay consists of .beta.-decay with an E.sub.mean larger than 0.6 MeV and a half-life greater than a few days qualify. In addition to these criteria, other criteria also play a role, e.g., scarcity of the starting material, the toxicity thereof, its mechanical processibility, etc.
In practice, therefore, there is a great need for a source of radioactive material with a .beta.-energy and a half-life that can be employed for vascular brachytherapy without difficulty, and that is inexpensive and easy to process mechanically.
A preferred radioactive source comprises a starting material that after activation produces a radioactive material that decays with nonclinically relevant radioactive decay to a decay product that in turn decays with clinically relevant .beta.-decay. In this way, a "combination source" is provided that is very suitable as .beta.-emitter in intravascular brachytherapy.
In this way, a much greater freedom of choice is obtained with regard to the starting material to be activated, particularly in terms of its cost price, mechanical processibility, toxicity, activability, etc.
The starting material, enriched or not, is activated in a reactor through neutron irradiation. By enriched starting material is understood an isotopic ratio that is changed in favor of one isotope, when compared to the isotopic ratio present in the natural starting material.
In an advantageous manner, the half-life of the activated starting material for a radioactive source is considerably greater than the half-life of the decay product.
More specifically, the half-life of the activated material can exceed 50 hours, while the half-life of the decay product then is less than 10 days. The half-life of the activated material is preferably at least ten times as great as the half-life of the decay product. In this way, a balance is reached relatively quickly between the production and the decay of the decay product. It is known, per se, that such a balance is reached only after about four times the half-life of the decay product. The half-life of the decay product should therefore preferably be less than ten days.
Tungsten is a particularly suited starting material. Natural tungsten consists of two stable isotopes W-184 and W-186. W-186 can be activated through neutron irradiation to W-188 through double neutron capture. In a reactor with sufficient high neutron flux, the double neutron capture should occur with sufficient frequency so as to generate a sufficiently high activity for this application (intravascular brachytherapy).
It should be noted that on account of its 69.4 day half-life, W-188 in principle would qualify as a radioactive source. However, as can be seen from the known tables, the .beta.-energy of the .beta.-radiation emitted by W-188 is much too low, i.e. 0.1 MeV, for it to be considered a likely candidate. Until now, W-188, being difficult to obtain (double neutron capture), was therefore dismissed as a radioactive source. Furthermore, as appears from the same tables, a radioactive material such as Re-188 qualifying in principle because of its .beta.-energy, is not applicable in practice, since its 17-hour half-life is much too short for the present application. And there are still many examples of radioactive materials that satisfy one but not the other criterium.
An especially advantageous radioactive source has tungsten, enriched or not, as a starting material which after activation produces W-188, with the decay product being Re-188.
In this way, a radioactive source is realized, in which Re-188 (which emits the clinically relevant .beta.-radiation) is not produced in advance in a reactor, but is produced rather on-site and continuously through the disintegration of W-188. With regard to the short 17-hour half-life of Re-188, the disintegration time of W-188 to Re-188 is so long (69.4 days) that a continuous production of Re-188 takes place and that apparently a Re-188 source is present with a half-life of more than 69 days, instead of only 17 hours. In this way, therefore, a source is obtained that satisfies both requirements of sufficiently high .beta.-energy and sufficient long half-life. Also a source is obtained of a material, tungsten, having excellent mechanical properties and being very well known through its use as filaments in incandescent lamps.
Specifically, application of a source can take place as a radioactive element that is to be attached to one extremity of an elongated wire-shaped element.
Applying a radioactive element attached to the extremity of an elongated wire-shaped element is known per se from European Patent Application EP-A-0433011.
In order to bring the radioactive element to the stretched place, it must be pushed, at least in part, through a catheter in a blood vessel. In particular, the short turns in the blood vessels of the coronary artery around the heart make a certain flexibility of the element necessary. In order to permit this displacement through a catheter in a blood vessel also to take place flexibly, the radioactive element, which may have the length of a few centimeters, must be sufficiently flexible and pliable in order to be able to slide without problems through the turns of a blood vessel.